Treadmill-trotting fly's ear drums offer hope for hearing aids

Ithaca, NY—A small, tree-dwelling fly called Ormia ochracea has directional hearing that's accurate to within one to two degrees. That's the same as humans, but in a much, much smaller package and with different mechanical functionality—providing inspiration to research scientists at Cornell University and to designers now trying to reverse-engineer Ormia's ear to improve directional hearing for wearers of hearing aids.

Ormia depends on superb hearing. To reproduce, the female Ormia needs a hearing organ that can locate singing male crickets. She deposits her larvae in the cricket, which serves as host for the developing flies. To avoid being detected and kicked off the cricket, the fly lands close to it and walks the last few steps. This behavior proved handy for scientists at Cornell, led by Ron R. Hoy, who put Ormia on a fly-sized treadmill to test its sound localization. (To see a QuickTime movie of the fly on the treadmill, visit www.news.cornell.edu/releases/March01/flyball.mov.)

"Generally, hearing aids that sell for under $1,000 have omni-directional microphones with no selectivity," Hoy says. "But human ears are directional, and tuned more to sounds coming from the front than from the sides and back. To get directional hearing aids now, wearers need multiple microphones and an on-board computer. These are cumbersome, unattractive and cost around $5,000."

But directionality based on the Ormia fly comes from the mechanics of the ear drum itself. "The fly's ear drums are linked together with a membrane-like cuticle. The ear drum nearest the sound vibrates a little ahead of, and more strongly than, the other," says Hoy. "There's a very small delay for sound at 90° [to the side]. For humans, the time delay is 100 microseconds, for the fly, just 1.5 microseconds—delayed to 55 microseconds by the mechanics of the ears." The delay is important, because sensory organs that detect vibration are attached directly to the fly's ear drum—independent of computation. "This fly gives us a unique mechanical system for extracting direction of sound," Hoy continues. "Engineers now need to build artificial ear drums out of silicon, with nano-fabrication technology, that mimic those of the fly."

Human directional hearing is accepted to be the best on earth, although Hoy says that it's hard to tell, because animals can't usually tell us where sound is coming from. "That's why the treadmill is very important. It's a ping-pong ball held up in an air current and painted with dots so a computer can calculate the fly's directional movement. We play recorded cricket chirps all around the fly, and it obliges us by walking in the direction of the chirp."

Researchers are currently working on mechanically coupled small wafers that don't need heavy computation, but rather take the differential signal and feed it to a hearing device. "If this can be done on silicon, it will be really inexpensive," Hoy says. A current prototype exists, but not at the right frequencies. Hoy says they hope to have something effective within five years, something with which manufacturers can work. (For a related story about hearing aid design, see Electronics lend an ear, DN, 6/4/01, p. 72)

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